Integral membrane proteins (IMPs) are crucial cellular components, mediating the transfer of material and signals between the extracellular environment and the cytoplasm, or between different cellular compartments. Structural and functional analysis of IMPs is important to furthering our understanding of membrane protein interactions. More than half of current pharmaceutical agents target proteins in this class. IMP characterization is often challenging, and sometimes impossible, because of difficulties associated with handling these macromolecules. IMPs in the native state display large hydrophobic surfaces, which are not compatible with an aqueous environment. Detergents are therefore required to extract IMPs from the lipid bilayer and to maintain the native state of the protein in solution. Nonionic detergents, such as dodecyl-β-D-maltoside (DDM) and octyl-β-D-glucoside (OG), are commonly used for these extractions. Despite the comparatively mild nature of DDM, OG and related detergents, many membrane proteins denature and/or aggregate upon solubilization with these agents.
Diverse strategies have been pursued to develop new tools for solubilization of IMPs from membranes and for maintenance of these proteins in a native-like state in aqueous solution. Unfortunately, techniques that are effective for solubilization are not always optimal or effective for stabilization, and vice versa. Strategies for developing new IMP tools have included exploration of small amphiphilic molecules that depart from traditional detergent architectures. Small amphiphiles that facilitate IMP crystallization are particularly noteworthy (see Chae et al., Nat. Methods 2010, 7, 1003-1008; Hovers et al., Mol. Membr. Biol. 2011, 28, 170; Rasmussen, et al., Nature 2011, 469, 236-240; Rosenbaum et al., Nature 2011, 469, 175-180; Rasmussen et al., Nature 2011, doi: 10.1038/nature10361).
Amphiphilic polymers (“amphipols”) and discoidal lipid bilayers stabilized by an amphiphilic protein scaffold (“nanodiscs”) represent highly innovative approaches for stabilizing IMPs in native-like states in aqueous solution. It is not clear, however, whether either of these approaches can support growth of high-quality crystals for diffraction analysis. Furthermore, neither amphipols nor nanodiscs were designed to extract IMPs from biological membranes. Despite considerable progress in the development of new compounds and strategies for membrane protein solubilization and stabilization, new tools are needed, because many IMPs are currently refractory. Given the great variation in structure and physical properties among membrane proteins, it is very unlikely that a single amphiphile or amphiphile family will be optimal for every system, or even most systems.
Accordingly, there is a need for new classes of structurally novel amphiphiles that display favorable behavior, relative to traditional detergents such as DDM, toward a diverse set of membrane proteins. There is also a need for novel amphiphiles that can aid membrane protein manipulation techniques such as solubilization, isolation, stabilization, and crystallization.